The prior known coating processes have conventionally been carried out by dissolving film-forming materials in a volatile solvent, applying the solution to a substrate and drying and curing the coating with the release of the solvent. Commonly, heat is applied to accelerate the film formation. The release of the solvent frequently leaves pinholes in the resulting coating. Also, the release of such solvents into the atmosphere is environmentally unacceptable.
More recently, solvent-free prepolymers such as epoxy resins and modified epoxy resins such as acrylic esters of polyepoxides, have been developed. Many of such formulations employ reactive diluents to adjust the viscosity of the composition to that suitable for casting or other fabrication techniques. Also, many of those compositions are photocurable which further minimizes the evolution of solvents. However, when those materials, such as the epoxyesters, are cured by ultraviolet light using the known photoinitiators and photosensitizers, the coatings are found to have unacceptable adhesion to metallic substrates such as aluminum. Adequate adhesion is attained in some cases only with a post thermal bake at elevated temperatures. In many cases even such a post cure is not sufficient to cause adhesion. Such a step is undesirably costly, tends to defeat the purpose of preventing loss of volatiles to the atmosphere and requires a high energy input. In addition, many of these systems tend to be air inhibited which may result in tacky surfaces or require extra cure time to produce a surface free of tack.
Radiation curable epoxy resins have been described which can provide excellent adhesion to metals. These systems suffer from a number of disadvantages, however. They cannot be readily cured in films greater than approximately 0.1 to 0.3 mil without wrinkling of the coating. Thicker coatings may appear cured but removal of the thin skin on top will reveal uncured resin underneath. An additional problem is frequently encountered with pure epoxy systems, especially those based exclusively on bisphenol diglycidyl ethers or epoxidized cycloolefins. Typically these products exhibit excessive brittleness which results in a low reverse impact on metal panels.
It would be desirable to have a coating formulation that is curable by ultraviolet light that retains the properties of prior known coatings and also exhibits improved adhesion to nonporous substrates such as metals and without the air inhibition exhibited by many conventional unsaturated ultraviolet curable compositions. It would also be desirable to obtain the useful properties of photocurable epoxies.